Conductive conductivity sensors are utilized in various applications for measuring conductivity of a medium.
The best known conductive conductivity sensors are the so called two electrode or four electrode sensors.
Two electrode sensors have two electrodes immersed in the medium and supplied with an alternating voltage during measurement operation. A measuring electronics connected to the two electrodes measures an electrical impedance of the conductivity measurement cell, from which a specific resistance or a specific conductance of the medium located in the measuring cell is then ascertained based on a cell constant determined earlier by the geometry and the character of the given measuring cell.
Four electrode sensors have four electrodes immersed in the medium during measurement operation, of which two electrodes are operated as so called electrical current electrodes and two electrodes are operated as so called voltage electrodes. During measurement operation, an alternating voltage is placed between the two electrical current electrodes, and therewith an alternating electrical current is fed into the medium. The electrical current fed in effects a potential difference lying between the voltage electrodes. The potential difference is preferably ascertained using a currentless measurement. Also here, by means of a measuring electronics connected to the electrical current electrodes and voltage electrodes, the impedance of the conductivity measurement cell is ascertained from the alternating electrical current fed in and the measured potential difference. Then, from the impedance, a specific resistance or a specific conductance of the medium located in the measuring cell is ascertained based on a cell constant determined earlier and resulting from the geometry and the character of the measuring cell.
During measurement operation, electric fields, whose field lines regularly extend out from the inner space of the measuring cell, form between the electrodes.
If such a conductivity sensor is operated in the vicinity of another object, such as e.g. a container wall, there is an influencing of the electric field lines by the object, and therewith an influencing of the measurement. This influencing, which is essentially dependent on the geometric arrangement of the object, its separation from the sensor, and its electrical conductivity, is regularly determined today using a complicated calibration method at the location of use, and therefrom, for example, an installation factor is determined, with which the influencing can be compensated in the following measurements.
Moreover, DE 10 2008 054 659 A1 describes reducing the influence of objects located in the environment of a conductive conductivity sensor by arranging planar electrodes parallel to one another on mutually opposing inner surfaces of an externally accessible cavity in an electrically insulating probe body.
This offers the advantage that the field lines extend essentially within the cavity between the electrodes, compared to conductivity sensors having electrodes lying at least partially completely free or protruding out from a probe body, such that they include a distinct free field outside of the probe body.
Also here, the spatial region occupied by the field lines is, however, clearly greater than the inner space of the cavity enclosed by the oppositely lying, flat electrodes.
In order to prevent an influencing by objects in the vicinity of the conductivity sensor, it is necessary to arrange the electrodes with a sufficient separation from the outwardly facing openings of the inner space. Among other things, this results in a certain minimum structural size of the conductivity sensors. It is not possible, without more, to consider using sizes below this minimum.